diff --git a/Makefile b/Makefile
index 50b260d62c..cfbf4e79fd 100644
--- a/Makefile
+++ b/Makefile
@@ -139,7 +139,206 @@ test: $(TEST)
 iwyu: build/.ran-cmake
 	cmake --preset iwyu
 	cmake --build build > build/iwyu.log
-	iwyu-fix-includes --only_re="src/nvim" --ignore_re="src/nvim/(auto|map.h|eval/encode.c)" --safe_headers < build/iwyu.log
+	iwyu-fix-includes --only_re="src/nvim" --ignore_re="(src/nvim/eval/encode.c|src/nvim/auto/\
+	|src/nvim/api/autocmd.h\
+	|src/nvim/api/buffer.h\
+	|src/nvim/api/command.h\
+	|src/nvim/api/deprecated.h\
+	|src/nvim/api/extmark.h\
+	|src/nvim/api/keysets.h\
+	|src/nvim/api/options.h\
+	|src/nvim/api/private/converter.h\
+	|src/nvim/api/private/defs.h\
+	|src/nvim/api/private/dispatch.h\
+	|src/nvim/api/private/helpers.h\
+	|src/nvim/api/private/validate.h\
+	|src/nvim/api/tabpage.h\
+	|src/nvim/api/ui.h\
+	|src/nvim/api/vim.h\
+	|src/nvim/api/vimscript.h\
+	|src/nvim/api/win_config.h\
+	|src/nvim/api/window.h\
+	|src/nvim/arabic.h\
+	|src/nvim/arglist.h\
+	|src/nvim/arglist_defs.h\
+	|src/nvim/ascii.h\
+	|src/nvim/assert.h\
+	|src/nvim/autocmd.c\
+	|src/nvim/autocmd.h\
+	|src/nvim/base64.h\
+	|src/nvim/buffer.h\
+	|src/nvim/buffer_defs.h\
+	|src/nvim/buffer_updates.h\
+	|src/nvim/bufwrite.h\
+	|src/nvim/change.h\
+	|src/nvim/channel.h\
+	|src/nvim/charset.h\
+	|src/nvim/cmdexpand.h\
+	|src/nvim/cmdexpand_defs.h\
+	|src/nvim/cmdhist.h\
+	|src/nvim/context.h\
+	|src/nvim/cursor.h\
+	|src/nvim/cursor_shape.h\
+	|src/nvim/debugger.h\
+	|src/nvim/decoration.h\
+	|src/nvim/decoration_defs.h\
+	|src/nvim/decoration_provider.h\
+	|src/nvim/diff.h\
+	|src/nvim/digraph.h\
+	|src/nvim/drawline.h\
+	|src/nvim/drawscreen.h\
+	|src/nvim/edit.h\
+	|src/nvim/eval.h\
+	|src/nvim/eval/buffer.h\
+	|src/nvim/eval/decode.h\
+	|src/nvim/eval/encode.h\
+	|src/nvim/eval/executor.h\
+	|src/nvim/eval/funcs.h\
+	|src/nvim/eval/gc.h\
+	|src/nvim/eval/typval.h\
+	|src/nvim/eval/typval_defs.h\
+	|src/nvim/eval/typval_encode.h\
+	|src/nvim/eval/userfunc.h\
+	|src/nvim/eval/vars.h\
+	|src/nvim/eval/window.h\
+	|src/nvim/event/libuv_process.h\
+	|src/nvim/event/loop.h\
+	|src/nvim/event/multiqueue.h\
+	|src/nvim/event/process.h\
+	|src/nvim/event/rstream.h\
+	|src/nvim/event/signal.h\
+	|src/nvim/event/socket.h\
+	|src/nvim/event/stream.h\
+	|src/nvim/event/time.h\
+	|src/nvim/event/wstream.h\
+	|src/nvim/ex_cmds.h\
+	|src/nvim/ex_cmds2.h\
+	|src/nvim/ex_cmds_defs.h\
+	|src/nvim/ex_docmd.h\
+	|src/nvim/ex_eval.h\
+	|src/nvim/ex_eval_defs.h\
+	|src/nvim/ex_getln.h\
+	|src/nvim/ex_session.h\
+	|src/nvim/extmark.h\
+	|src/nvim/extmark_defs.h\
+	|src/nvim/file_search.h\
+	|src/nvim/fileio.h\
+	|src/nvim/fold.h\
+	|src/nvim/fold_defs.h\
+	|src/nvim/garray.h\
+	|src/nvim/getchar.h\
+	|src/nvim/getchar_defs.h\
+	|src/nvim/globals.h\
+	|src/nvim/grid.h\
+	|src/nvim/grid_defs.h\
+	|src/nvim/hashtab.h\
+	|src/nvim/help.h\
+	|src/nvim/highlight.h\
+	|src/nvim/highlight_defs.h\
+	|src/nvim/highlight_group.h\
+	|src/nvim/iconv.h\
+	|src/nvim/indent.h\
+	|src/nvim/indent_c.h\
+	|src/nvim/input.h\
+	|src/nvim/insexpand.h\
+	|src/nvim/keycodes.h\
+	|src/nvim/linematch.h\
+	|src/nvim/log.h\
+	|src/nvim/lua/base64.h\
+	|src/nvim/lua/converter.h\
+	|src/nvim/lua/executor.h\
+	|src/nvim/lua/secure.h\
+	|src/nvim/lua/spell.h\
+	|src/nvim/lua/stdlib.h\
+	|src/nvim/lua/treesitter.h\
+	|src/nvim/lua/xdiff.h\
+	|src/nvim/macros.h\
+	|src/nvim/main.h\
+	|src/nvim/map.h\
+	|src/nvim/mapping.h\
+	|src/nvim/mapping_defs.h\
+	|src/nvim/mark.h\
+	|src/nvim/mark_defs.h\
+	|src/nvim/marktree.h\
+	|src/nvim/match.h\
+	|src/nvim/mbyte.h\
+	|src/nvim/mbyte_defs.h\
+	|src/nvim/memfile.h\
+	|src/nvim/memfile_defs.h\
+	|src/nvim/memline.h\
+	|src/nvim/memory.h\
+	|src/nvim/menu.h\
+	|src/nvim/message.h\
+	|src/nvim/mouse.h\
+	|src/nvim/move.h\
+	|src/nvim/msgpack_rpc/channel.h\
+	|src/nvim/msgpack_rpc/channel_defs.h\
+	|src/nvim/msgpack_rpc/helpers.h\
+	|src/nvim/msgpack_rpc/server.h\
+	|src/nvim/msgpack_rpc/unpacker.h\
+	|src/nvim/normal.h\
+	|src/nvim/nvim/extmark.h\
+	|src/nvim/ops.h\
+	|src/nvim/option.h\
+	|src/nvim/option_defs.h\
+	|src/nvim/option_vars.h\
+	|src/nvim/optionstr.h\
+	|src/nvim/os/dl.h\
+	|src/nvim/os/fileio.h\
+	|src/nvim/os/fs.h\
+	|src/nvim/os/input.h\
+	|src/nvim/os/lang.h\
+	|src/nvim/os/os.h\
+	|src/nvim/os/process.h\
+	|src/nvim/os/pty_conpty_win.h\
+	|src/nvim/os/pty_process_unix.h\
+	|src/nvim/os/pty_process_win.h\
+	|src/nvim/os/shell.h\
+	|src/nvim/os/time.h\
+	|src/nvim/os/tty.h\
+	|src/nvim/path.h\
+	|src/nvim/plines.h\
+	|src/nvim/popupmenu.h\
+	|src/nvim/profile.h\
+	|src/nvim/quickfix.h\
+	|src/nvim/regexp.h\
+	|src/nvim/regexp_defs.h\
+	|src/nvim/runtime.h\
+	|src/nvim/search.h\
+	|src/nvim/sha256.h\
+	|src/nvim/shada.h\
+	|src/nvim/sign.h\
+	|src/nvim/sign_defs.h\
+	|src/nvim/spell.h\
+	|src/nvim/spell_defs.h\
+	|src/nvim/spellfile.h\
+	|src/nvim/spellsuggest.h\
+	|src/nvim/state.h\
+	|src/nvim/statusline.h\
+	|src/nvim/statusline_defs.h\
+	|src/nvim/strings.h\
+	|src/nvim/syntax.h\
+	|src/nvim/tag.h\
+	|src/nvim/terminal.h\
+	|src/nvim/testing.h\
+	|src/nvim/textformat.h\
+	|src/nvim/textobject.h\
+	|src/nvim/tui/input.h\
+	|src/nvim/tui/terminfo.h\
+	|src/nvim/tui/tui.h\
+	|src/nvim/ugrid.h\
+	|src/nvim/ui.h\
+	|src/nvim/ui_client.h\
+	|src/nvim/ui_compositor.h\
+	|src/nvim/undo.h\
+	|src/nvim/undo_defs.h\
+	|src/nvim/usercmd.h\
+	|src/nvim/version.h\
+	|src/nvim/vim.h\
+	|src/nvim/viml/parser/expressions.h\
+	|src/nvim/viml/parser/parser.h\
+	|src/nvim/window.h\
+	|src/nvim/winfloat.h)" --nosafe_headers < build/iwyu.log
 	cmake -B build -U ENABLE_IWYU
 	cmake --build build
 
diff --git a/src/clint.py b/src/clint.py
index 63182a4349..cda7c0e580 100755
--- a/src/clint.py
+++ b/src/clint.py
@@ -151,6 +151,7 @@ Syntax: clint.py [--verbose=#] [--output=vs7] [--filter=-x,+y,...]
 _ERROR_CATEGORIES = [
     'build/endif_comment',
     'build/header_guard',
+    'build/include_defs',
     'build/printf_format',
     'build/storage_class',
     'readability/bool',
@@ -879,6 +880,196 @@ def CheckForHeaderGuard(filename, lines, error):
         error(filename, 0, 'build/header_guard', 5,
               'No "#pragma once" found in header')
 
+def CheckIncludes(filename, lines, error):
+    """Checks that headers only include _defs headers
+
+    Args:
+      filename: The name of the C++ header file.
+      lines: An array of strings, each representing a line of the file.
+      error: The function to call with any errors found.
+    """
+    if filename.endswith('.c.h') or filename.endswith('.in.h') or FileInfo(filename).RelativePath() in {
+        'func_attr.h',
+    }:
+        return
+
+    check_includes_ignore = [
+            "api/command.h",
+            "api/deprecated.h",
+            "api/extmark.h",
+            "api/keysets.h",
+            "api/options.h",
+            "api/private/converter.h",
+            "api/private/defs.h",
+            "api/private/dispatch.h",
+            "api/private/helpers.h",
+            "api/private/validate.h",
+            "api/tabpage.h",
+            "api/ui.h",
+            "api/vim.h",
+            "api/vimscript.h",
+            "api/win_config.h",
+            "arglist.h",
+            "arglist_defs.h",
+            "ascii.h",
+            "assert.h",
+            "autocmd.h",
+            "buffer.h",
+            "buffer_defs.h",
+            "buffer_updates.h",
+            "change.h",
+            "channel.h",
+            "charset.h",
+            "cmdexpand.h",
+            "cmdexpand_defs.h",
+            "cmdhist.h",
+            "context.h",
+            "cursor.h",
+            "cursor_shape.h",
+            "decoration.h",
+            "decoration_defs.h",
+            "decoration_provider.h",
+            "diff.h",
+            "digraph.h",
+            "drawline.h",
+            "drawscreen.h",
+            "edit.h",
+            "eval.h",
+            "eval/decode.h",
+            "eval/encode.h",
+            "eval/funcs.h",
+            "eval/typval.h",
+            "eval/typval_defs.h",
+            "eval/typval_encode.h",
+            "eval/userfunc.h",
+            "event/libuv_process.h",
+            "event/loop.h",
+            "event/multiqueue.h",
+            "event/process.h",
+            "event/rstream.h",
+            "event/signal.h",
+            "event/socket.h",
+            "event/stream.h",
+            "event/time.h",
+            "event/wstream.h",
+            "ex_cmds.h",
+            "ex_cmds_defs.h",
+            "ex_docmd.h",
+            "ex_eval_defs.h",
+            "ex_getln.h",
+            "extmark_defs.h",
+            "file_search.h",
+            "fileio.h",
+            "fold.h",
+            "fold_defs.h",
+            "garray.h",
+            "getchar.h",
+            "getchar_defs.h",
+            "globals.h",
+            "grid.h",
+            "grid_defs.h",
+            "hashtab.h",
+            "highlight.h",
+            "highlight_defs.h",
+            "highlight_group.h",
+            "iconv.h",
+            "indent.h",
+            "indent_c.h",
+            "input.h",
+            "insexpand.h",
+            "keycodes.h",
+            "linematch.h",
+            "log.h",
+            "lua/converter.h",
+            "lua/executor.h",
+            "lua/treesitter.h",
+            "macros.h",
+            "main.h",
+            "map.h",
+            "mapping.h",
+            "mapping_defs.h",
+            "mark.h",
+            "mark_defs.h",
+            "marktree.h",
+            "mbyte.h",
+            "mbyte_defs.h",
+            "memfile_defs.h",
+            "memline.h",
+            "memory.h",
+            "menu.h",
+            "message.h",
+            "mouse.h",
+            "move.h",
+            "msgpack_rpc/channel_defs.h",
+            "msgpack_rpc/helpers.h",
+            "msgpack_rpc/unpacker.h",
+            "normal.h",
+            "nvim/extmark.h",
+            "ops.h",
+            "option.h",
+            "option_defs.h",
+            "option_vars.h",
+            "os/fileio.h",
+            "os/fs.h",
+            "os/input.h",
+            "os/lang.h",
+            "os/os.h",
+            "os/process.h",
+            "os/pty_conpty_win.h",
+            "os/pty_process_unix.h",
+            "os/pty_process_win.h",
+            "os/shell.h",
+            "path.h",
+            "plines.h",
+            "popupmenu.h",
+            "profile.h",
+            "quickfix.h",
+            "regexp.h",
+            "regexp_defs.h",
+            "runtime.h",
+            "search.h",
+            "sha256.h",
+            "sign_defs.h",
+            "spell.h",
+            "spell_defs.h",
+            "spellfile.h",
+            "spellsuggest.h",
+            "statusline.h",
+            "statusline_defs.h",
+            "strings.h",
+            "syntax.h",
+            "tag.h",
+            "textformat.h",
+            "textobject.h",
+            "tui/input.h",
+            "tui/terminfo.h",
+            "tui/tui.h",
+            "ugrid.h",
+            "ui.h",
+            "ui_client.h",
+            "ui_compositor.h",
+            "undo_defs.h",
+            "usercmd.h",
+            "version.h",
+            "vim.h",
+            "viml/parser/expressions.h",
+            "viml/parser/parser.h",
+            "window.h",
+            "winfloat.h",
+                             ]
+
+    for i in check_includes_ignore:
+        if filename.endswith(i):
+            return
+
+    for i, line in enumerate(lines):
+        if("#include" in line):
+            if("#include <" in line):
+                continue
+            if "_defs" not in line:
+                error(filename, i, 'build/include_defs', 5,
+                      'Headers should not include non-"_defs" headers')
+
 
 def CheckForBadCharacters(filename, lines, error):
     """Logs an error for each line containing bad characters.
@@ -2166,6 +2357,7 @@ def ProcessFileData(filename, file_extension, lines, error,
 
     if file_extension == 'h':
         CheckForHeaderGuard(filename, lines, error)
+        CheckIncludes(filename, lines, error)
 
     RemoveMultiLineComments(filename, lines, error)
     clean_lines = CleansedLines(lines, init_lines)